Many applications which require some source of heat utilize alpha-emitting radioactive fuel elements or capsules which are fueled with radioisotopes such as plutonium-238, polonium-210, curium-244, americium-241, neptunium-237 or the like. These isotopic fuel elements or capsules may be used for thermoelectric, thermionic, and other power conversion units or simply as heat sources for space or the like applications. In the alpha decay of these radioisotopes, alpha particles are emitted which in turn may acquire electrons to become complete helium molecules. Such helium or alpha particles, in time, may build up undesirably high pressures within the fuel element or capsule containment vessel. In many applications, a radioactive fuel element or capsule may operate at elevated temperatures, such as at about 3,600.degree.F and higher, which may further increase helium pressure within the heat source container and cause material weakening of the housing of containment vessel walls. Consequently, materials which have high strength at high temperatures are used for enclosing such alpha and helium producing radioisotopes and frequently, double walls of thick, high strength material are desirable to minimize or prevent rupture and escape of the radioactive isotopes. The additional weight resulting from the use of such containment vessels may present a difficult problem in rocket and space vehicle applications as well as other uses where excess weight is undesirable.
The helium so generated may be vented or released from the containment vessel to a pressure chamber and/or to the atmosphere. Under normal usage, the helium may be allowed to escape while radioactive elements in the form of fines or other particles must be contained within the containment vessel. A vent or vent system must not only withstand the high temperature and possibly high pressure resulting from the radioactive decay process itself, but must also in space applications and the like be capable of withstanding all possible launch pad environments, such as fireball, heat pulse, explosion, propellant burn, and launch pad impact, and be capable of withstanding reentry impact, vibration, thermal stresses, and corrosion.
Prior radioisotope fuel elements or capsules have employed various porous materials as vents for release of the helium produced therein. Many of these porous materials are of inherently low strength or are unable to withstand the high operating temperatures. Still other materials have thermal characteristics, such as coefficients of thermal expansion, significantly different from the containment vessel materials so that they are subject to rupture or to other failures when subjected to a heat pulse or the like. Still other materials are not able to withstand the corrosive environment presented by the radioisotopic material itself. Further, it has been difficult in many prior vents to reliably predict the porosity and vent rate of the vent or have been very difficult and expensive to manufacture.